Recent advances have brought near-field scanning optical microscopy (NSOM) to the point where it can be applied routinely to a variety of samples. For example, the design and applications of a probe based on a metal-coated, tapered optical fiber are described in E. Betzig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R. L. Kostelak, Science 25 1, 1468 (1991); E. Betzig, P. L. Finn, and J. S. Weiner, Appl, Phys. Lett. 60, 2484 (1992); and E. Betzig and J. K. Trautman, Science 257, 189 (1992). Nevertheless, further refinement of near-field probes remains an area of active interest. For example, the quantity and diversity of applications would be enhanced through the development of probes having increased photon flux. In the above-mentioned tapered fiber probe, the flux is limited, in large part, because the transmitted energy is exponentially attenuated in evanescent modes within the probe as the probe diameter tapers to dimensions substantially smaller than the wavelength. However, rather than imaging the sample directly via this comparatively weak emitted light, it is possible, in principle, to measure local properties of the sample by their influence on the boundary conditions at the emissive aperture of the probe and their consequent effect on the electromagnetic field within the probe itself. The problem then becomes one of measuring these field changes with sufficient speed and sensitivity to permit high bandwidth NSOM reflection-mode detection.
Reflective feedback probes have, in fact, been demonstrated for operation at heights greater than one wavelength above the sample surface. For example, U.S. Pat. No. 4,860,276, issued to H. Ukita, et al. on Aug. 22, 1989, describes an optical head which can be used for reading or writing digital data. This head, which is carded on a flying slider, includes a self-coupled semiconductor laser situated within several micrometers of the recording surface. The resulting spot size is about 1 .mu.m in diameter, which, however,is not substantially smaller than spot sizes attainable using conventional focusing optics. Thus, the Ukita probe fails to combine the high resolution of near-field detection with the relatively high signal-to-noise ratios achievable by reflective feedback.